Cleaning circuit boards before conformal coating is a huge topic by itself. This is because cleaning circuit boards can be challenging.
However, successful cleaning of electronic circuits can be achieved by a variety of techniques.
The main methods of cleaning can include:
• Aqueous wash
• Semi-aqueous wash
• Solvent & chemical wash
• Plasma cleaning
The key to success in cleaning circuit boards is similar to the success made with conformal coating. You need to match the cleaning process, the cleaning materials and the circuit board together.
If you do this then this will give you the best results for cleaning the circuit board assembly.
Why clean circuit boards before conformal coating?
The cleaning of a printed circuit board (PCB) before conformal coating application is normally done for two key reasons:
These are:
• Contamination removal
• Process improvement
They have different effects on the lifetime of the circuit board but can be equally important.
What types of contamination may be present on a circuit board?
Cleaning is used to remove many different types of contaminants from the manufacturing and assembly processes.
The residues can come from:
• Board laminate manufacture
• Component manufacture
• Soldering assembly processes (fluxes)
• Glue and ruggedizing processes
• Operator handling (finger prints, hair)
• Machine contamination (oils and greases)
• Environmental contamination (dust)
Removing the contamination may be a priority depending on their harmfulness.
Need to find out more?
Click conformal coating cleaning for further information or contact us directly and we can help you.
If you are new to Nexus and our work on conformal coatings then a good place to go is our Start Here page or our free conformal coating eBook.
Selective spraying of conformal coating using an automated robot system is one of the widest used application methods in high volume processing.
The principle is that the conformal coating is applied selectively by a small spray gun to the circuit board to the areas requiring coating only.
The selective process deliberately does not apply the conformal coating to areas that normally require masking such as connectors and other components.
This selective application of the conformal coating to the circuit board can avoid using a time consuming masking process and costs can normally be reduced.
What equipment do you need for selective spraying?
A specialist robotic system designed for application of conformal coatings is normally required for selective coating.
So, you are considering outsourcing your conformal coating or Parylene process to a subcontract supplier.
What’s the next step?
Assuming you have decided this is the right choice then the next task is to choose the right service provider.
This can be as difficult as selecting the right PCB manufacturer or laminate provider. There are good contractors and there are others. There are small providers and there are large-scale turnkey solutions.
The obvious answer is “the right one for me” but how do you ensure that you make the right decision?
Here are a few of the questions you should ask before signing up to the wrong coating solution provider and stop you wishing you had kept the coating process in house.
There are several questions you should ask before signing up to the wrong coating solution provider and stop you wishing you had kept the coating process in house!
The three key points to consider when choosing a subcontractor
When considering the supplier look at the three main areas.
The order you look at is up to you but ultimately you need to be happy in all three to keep sending the work out.
These three key areas are:
Quality
Turnaround time
Price
If you get these three key areas correct then you have succeeded. Everyone will be happy and there are no more problems with conformal coating.
However, within each of these areas, lie a lot of questions to be asked and if one of them isn’t working properly then who knows what the consequences are around the corner.
So, let’s consider each of these areas in turn in the order they are listed.
Quality
Everyone wants good quality. But what does that mean?
Whose quality are we judging against and how do we reach agreement?
One of the most important factors in subcontract conformal coating services is to agree what the PCB coating finish should look like.
Sound simple?
It can be if you define exactly what you want as a customer. The problem comes when you don’t know!
The reality is most customers have a preconceived idea of what a conformal coated printed circuit board should look like and the key for the customer is to communicate this to the provider.
Let me give you an example.
Consider a simple connector on a circuit board like the one below.
Then decide on what statement you agree with below:
Only the pins must not have coating on but the rest of the connector body does not matter.
The whole of the connector must have no coating on it all but there can be a gap of 1-2mm around it free of coating.
The whole of the connector must have no coating on it all and there can be no area around the connector free of coating.
All three options provide a connector free of coating. All three options work. All three options could be considered fine by various different customers.
However, the order they are stated is also lowest difficulty (aka cost) to highest difficulty to actually complete the work in a coating production line.
So, defining how you want to coat the board intimately impacts on the price of the project.
A key issue highlighted
Unfortunately, this highlights a key problem in conformal coating processing.
There are no standards that state what is the best solution and only guidelines. Ultimately, it comes down to a decision made by the customer as to how the PCB should be coated.
This means it is a crucial factor for both the customer and the supplier to define the level of quality. Get this right and most of your problems are sorted.
If the coating house is good they will help you define this from the beginning. They will not assume any level of quality but ask you what you need.
If they don’t help you then hope that they can guess what they need to provide you with.
A key problem in conformal coating processing is that there are no standards that state what is the best conformal coating finish and only guidelines. Ultimately, it comes down to a decision made by the customer as to how the PCB should be coated.
So what’s the next stage in quality?
So, you have agreed what quality of finish you want. Assuming that you know which conformal coating material you want then the rest should be easy.
Well, that statement is right as long as:
The supplier knows how to use your coating material correctly
Has the right equipment to apply the conformal coating
Has selected the right process for your PCB
Knows what to do when things go wrong
So, what you really need to do is find a subcontract conformal coating supplier that knows a lot about conformal coatings or Parylene.
Turnaround Time
So, you are considering outsourcing the coating work to an outside contractor but want the PCB coated when you need it.
Unfortunately, conformal coating is normally one of the last processes in a long line of operations so any delays in the manufacture of the PCB is normally being compounded by sending it out to a coating house.
Therefore, you need a fast turnaround option and your coating service should be flexible on this, allowing you choices on getting the PCBs coated.
However, you may want to consider the speed you require the PCB to be coated since it can lead to:
Extra costs
Potential problems with the process
Mistakes due to staff and machine availability
Availability of material
Local or global supply
Low cost offshore facilities
Capacity
These factors can influence the price significantly.
So, we have examined quality and turnaround time. The critical factor that ties these two areas together is price. Let’s take a look at this area.
Price
“I want this PCB coated for 45 cents”, says the customer.
“Okay can we see the board”, says the coating service provider.
The customer produces a 12”x6” PCB with 25 surface mount connectors with via’s everywhere and asks for coating both sides and wants all of the connectors not to be coated.
Okay, we have a mismatch in perception and this sounds ridiculous. But, it happens more regularly than it should.
Some customers have no idea of what it costs to coat a PCB. After all it’s just coating.
However, it’s up to the coating house to educate them so that they can get what they need and reach the happy point of all three areas satisfied.
So, what price should it be?
Well factors to be considered by the subcontractor are:
Material specified by the customer
Process to be used on the PCB
Volume of PCBs to be supplied
Amount of masking / keep out areas on the PCB
Amount of coating to be used per PCB
Is there cleaning involved before coating?
Yes cleaning is required, extra cost added in
No cleaning is not required, may be extra cost for finishing process if a lot of contamination
Speed of turnaround required (impact on resources, drying, curing)
Quality required (how much time finishing, inspecting, how close to look?)
So, the last two factors, turnaround and quality, tie directly back to the price and in reality have a huge impact.
Therefore, we find unsurprisingly all free factors should be considered as a whole and not separately.
Some customers have no idea of what it costs to conformal coat a circuit board. After all it’s just coating….
Summary
Subcontracting out your conformal coating process isn’t difficult if you look carefully at the three areas of quality, turnaround time and price.
If you can achieve all three with your subcontract coating house then you will be happy.
If you ignore one of the factors then it may be a less pleasant position.
Measuring the exact thickness of your conformal coating across the whole of a circuit board is not simple.
In fact, due to the geometry of the components and the fact that the coating is measured in microns, it makes this task almost impossible.
However, what you can do is measured the thickness of the conformal coating in a few key places and use the thickness information found to infer how the coating coverage is for the rest of the board.
This is how nearly all companies measure conformal coating thickness.
So, how is the conformal coating thickness measured on a circuit board?
There are several ways to measure the conformal coating thickness on a printed circuit board (PCB).
The methods used can be for either a dry or wet conformal coating.
These techniques include:
Non-destructive eddy current system
Micrometer screw gauge
Wet film gauge
These techniques are explored further below.
Non-destructive eddy current system
A fast method for measuring conformal coating thickness after drying is a system using eddy currents.
The process works by placing the test probe head flat on the surface of the conformal coating and a measurement taken.
The system provides an immediate repeatable result for thickness measurement of conformal coating.
The process is quick and accurate to ±1 um. Using a gauge and flying probe also means the measurement system is extremely easy to use.
Using a test probe system can quickly give you conformal coating thickness measurements without damaging the circuit board. Image from SCH Technologies
There are a couple of issues using an eddy current system like this.
First, there needs to be metal in the circuit board directly below the tested point. Otherwise, the system cannot function correctly as the eddy current will pass directly through the board.
Second, there needs to be a flat area on the board large enough for the test probe. The smallest practical probe is approximately 6mm diameter so any area smaller than this is not practical.
Finally, the surface measured for the probe needs to be flat. If not then there will be errors in the measurement. So, measuring components is extremely difficult.
To overcome these problems it may be better to measure test coupons.
Apply the conformal coating to the test coupons at the same time as the circuit board allows an easy measurement process. It also provides a permanent measurement.
In fact, test coupons are the ideal method for measuring the coating thickness, whatever the conformal coating process and method of measurement.
Micrometer screw gauge
An alternative to the eddy current system for dry film measurement is a calibrated micrometer screw gauge.
It’s a low cost, low-tech method for measuring conformal coating thickness and can normally measure down to ± 10 um.
The process is relatively simple.
First measure a point on the board or test coupon before coating. Next, apply the coating. Cure the coating well and finally re-measure at the same point.
The difference in the two measurements gives you the conformal coating thickness.
A couple of pitfalls to avoid with this technique are ensuring the conformal coating is cured hard enough since if it is soft it could compact and give a false reading.
Also, do not measure one point. Take an average of at least 3 or 4 points across the coupon since this will give a better result statistically.
Again, for this technique test coupons are the ideal method.
Wet film gauge
A final method that can be used is a wet coating measurement technique that is very cost effective.
The technique uses a comb with different size patterns that is placed in the wet conformal coating and the imprint left indicates the wet film thickness.
Knowing the solids content of the material means that the material thickness can be calculated.
A wet film gauge is a low cost method for measuring coating thickness while the conformal coating is wet. Using the solids content in the material and the wet film thickness allows the dry film thickness to be estimated.
Polyurethane (UR) conformal coatings generally provide good humidity & moisture protection although not always as good as the acrylics. However, it normally is enough to protect the circuit board.
Normally, urethanes are selected for their excellent chemical resistance. This is because the coatings cure rather than dry. That is they cross-link by one of many different methods including heat, UV, moisture and catalysed cure.
Typically they have higher dielectric properties compared to the acrylic conformal coatings.
Their chemical resistance, however, can be a limitation since rework and repair generally is more difficult than the acrylic coatings.
UR coatings are normally available as either single or two-component formulations. Pot life is dependent on the cure mechanism but can be more limited than the acrylic coatings.
The cleaning of a printed circuit board (PCB) before conformal coating application is normally done for two key reasons:
These are:
Contamination removal
Process improvement
They have different effects on the lifetime of the circuit board but can be equally important.
Contamination removal
The removal of contaminants from the surface of a printed circuit board before coating application could be important.
The contamination may be harmful and affect the long-term reliability of the circuit. Defects like corrosion can be devastating to a circuit performance in the field.
Also, applying the coating over the contamination will not necessarily improve the reliability.
Therefore, cleaning the PCB before coating could be advantageous.
Process improvement
Cleaning can aid the conformal coating application process.
It could avoid conformal coating defects like de-wetting and delamination. It can also help promote adhesion of the coating to the PCB.
The surface preparation can be extremely important, especially where some conformal coatings may have compatibility issues with the circuit board.
Therefore, cleaning a PCB before conformal coating may improve the coating quality and minimise defects.
What types of contamination may be present on a circuit board?
Cleaning is used to remove many different types of contaminants from the manufacturing and assembly processes.
They can affect the long-term reliability of the circuit after conformal coating.
The residues can come from many areas including:
Board laminate manufacture
Component manufacture
Soldering assembly processes (fluxes)
Glue and ruggedizing processes
Operator handling (finger prints, hair)
Machine contamination (oils and greases)
Environmental contamination (dust)
Removing the contamination may be a priority depending on their harmfulness.
How should I clean the printed circuit board?
Cleaning circuit boards before coating is a huge topic by itself.
Successful cleaning of circuits can be achieved by a variety of techniques.
These cleaning techniques include:
Aqueous
Semi-aqueous
Solvent & chemical
Plasma
The key to success in cleaning is similar to the success made with coating.
You need to match the cleaning process, materials and the circuit board together.
This will give the best results for application of the conformal coating.
How do I validate my cleaning process?
There are many techniques that can be used to measure cleanliness. They include many IPC test methods.
The techniques available include:
Ionic Contamination Testing (ROSE)
Ion Chromatography (IC) or High Performance Ion Chromatography (HPLC)
Surface Insulation Resistance (SIR) testing
Further information can be found in the IPC HDBK 001 on different cleanliness assessment methods or talk to us.
Masking boots are formed shapes used to cover components before conformal coating.
They are made to replace tape and dots in the conformal coating masking process for both Parylene and liquid conformal coating processes.
Using masking boots in protecting printed circuit boards (PCBs) from conformal coating application errors can be a lower cost, more reliable solution to using tapes.
The use of masking tapes, dots and liquid latex can be an effective process in protecting components from ingress of conformal coating.
However, the masking can be labour intensive, difficult and time consuming.
Masking boots can offer a labour saving alternative in both the masking and de-masking stages of the coating process. This saves time and money.
Further if the boots are recyclable then the savings can be even higher.
Three reasons to use masking boots
Masking time is normally reduced. Masking boots can be 4-5 times quicker than masking tape.
De-masking time is reduced. Again it is normally much quicker to remove masking boots.
Masking boots are much less likely to leak. So there is potentially less damage and rework of the circuit.
This means you could save a lot of money very quickly when switching to masking boots.
Masking boots can offer a labour saving alternative in both the masking and de-masking stages of the coating process. This saves time and money.
Measuring the exact thickness of your conformal coating across the whole of a circuit board is not simple.
Cleaning can aid the conformal coating application process.
Successful cleaning of circuits can be achieved by a variety of techniques.
Masking boots are formed shapes used to cover components before conformal coating.
